The present invention relates to an apparatus for measuring a deposition speed of electroless plating liquid.
Conventionally, in a manufacturing step of a printed control panel or distributing board, an electroless copper plating step occupies the important position. Importance of the electroless copper plating step increases more and more with development of an additive process. The reason for this is that, when a plating thickness is not set to an adequate value, defects occur in the quality aspect. For example, in the case where the plating thickness is large, a problem of etching leavings occurs in the subsequent pattern forming step. To the contrary, in the case where the plating thickness is conversely thin, problems such as occurrence of cracks and the like arise, which causes the through-hole reliability to be reduced.
In order to prevent such deficiencies, it is necessary to control the plating thickness to an adequate value. As a measuring method of controlling the plating thickness, there is a weight method in which a weight of plated copper, which is deposited for a predetermined period of time, for example, for thirty (30).about.sixty (60) minutes, is measured. Since, however, this measuring method must detect the weight of the copper periodically, there are disadvantages that it becomes difficult to continue the measurement, and to bring the measurement to an apparatus.
In view of the above, as a technique for removing the above-described disadvantages, in recent years, there has been proposed, in Japanese Laid Open Patent Publication No. SHO 58-141373 or the like, a method for measuring a deposition speed of the electroless plating liquid by means of a polarization resistance process. This is a method which is characterized in that, when electroless copper plating reaction advances on a pair of copper electrodes, in electroless plating liquid, under conditions that electrical potential is Ep1 and a deposition speed is 1p1, a constant or predetermined electric current .DELTA.I is given from the outside in accordance with the following relational expression, and an outside polarization .DELTA.E induced thereby is measured: ##EQU1## where .DELTA.E is the outside polarization;
.DELTA.I is the outside current; and PA1 K is a constant determined by the reaction condition.
In the method described above, however, not only is the apparatus complicated in structure and more expensive, but also the surface of the dissolved metal is apt to be oxidized. Accordingly, there are problems that it is difficult to effect plating onto the electrode surfaces, and it becomes difficult to effect correct or accurate measurement.
In fact, experiments conducted by the inventors of the present application enabled the results in which fluctuation of a measurement value was brought to .+-.20% or more per two (2) days, to be obtained.
In connection with the above, in the experiments, electroless copper plating liquid L-59 is used which is manufactured by Hitachi Chemical Co. Ltd.
As described above, when the fluctuation of the measurement value is brought to .+-.20% or more, it is not possible to control the correct or accurate plating thickness. Etching leavings and cracks occur in the measuring step of the printed control panel. This reduces the through-hole reliability considerably.